1. Field of the Invention
The present invention relates to a multi-layered organic electrophotographic photoconductor comprising an undercoat layer and principal functional layers of a charge generation layer and a charge transport layer, and particularly relates to an organic binder resin used in the charge generation layer.
2. Description of the Related Art
Various types of electrophotographic photoconductors have been developed since the invention of C. F. Carlson, U.S. Pat. No. 2,297,691. The electrophotographic photoconductors include inorganic photoconductors and organic photoconductors. An inorganic photoconductor uses an inorganic photoconductive material such as amorphous silicon, selenium, selenium-tellurium compound, selenium-arsenic compound, or zinc oxide. A type of an organic photoconductor comprises laminated layers of a charge generation layer and a charge transport layer. The charge generation layer contains charge generation material mainly composed of a photoconductive functional material of an organic pigment such as phthalocyanines or azo compounds that is dissolved or dispersed in an organic binder resin. The charge transport layer contains charge transport material functioning transporting electrons or holes generated in the charge generation layer upon receipt of light to the surface of the charge transport layer. The charge transport material is dissolved or dispersed in an organic binder resin. The charge generation layer and the charge transport layer are function-separated lamination-type organic thin films formed on a cylindrical conductive substrate using a coating liquid containing dissolved or dispersed charge generation or charge transport materials mentioned above.
In the case of an organic electrophotographic photoconductor of a function-separated type mentioned above, the charge generation layer and the charge transport layer are normally formed through an undercoat layer over a conductive substrate, and are occasionally laminated directly on the substrate. The underlayer is sometimes formed by an alumite layer that is an anodized film of aluminum, as in the case of an aluminum substrate. However, an inexpensive organic resin is often used for laminating the undercoat layer to conserve cost. The charge generation layer is a very thin film with a thickness of at most about 1 μm and is formed by dispersing the above-mentioned pigment particles in an organic binder resin. The charge transport layer is formed by dissolving a charge transport material with a relatively low molecular weight in an organic binder resin such as polycarbonate resin to provide a molecular dispersion state. The thickness of the charge transport layer is usually in the range from 10 μm to 30 μm.
Semiconductor lasers and light emitting diodes are often used in printers, digital copiers, facsimile machines, and digital image complexes that perform these functions together. The semiconductor laser and light emitting diodes emit light with a wavelength in the range from 635 to 780 nm, which is longer than the main wavelength of a white light source that is commonly used as a light source for photoconductors. Consequently, photoconductors having sensitivity to the light with such long wavelengths are needed and have been developed. For example, the phthalocyanines mentioned previously exhibit a larger value of absorbance in the wavelength range emitted by semiconductor lasers than other charge generation materials. In addition, the phthalocyanines exhibit an excellent charge generation capability in this wavelength range. Consequently, the phthalocyanines have been extensively studied for use as a charge generation material of photoconductors carried on above-mentioned apparatuses employing a light source of a semiconductor laser.
The known phthalocyanines that exhibit excellent charge generation capability in the long wavelength range include the compounds having a central metal of copper, aluminum, indium, vanadium, or titanium as disclosed in Japanese Unexamined Patent Application Publication Nos. S53-89433 and S57-145748, and U.S. Pat. Nos. 3,816,118 and 3,825,422.
In the apparatuses of analog copiers using a white light source, for example, a halogen lamp, mainly used are multi-layered organic electrophotographic photoconductors using a charge generation material of a bisazo compound having a sensitivity in the wavelength range of 400 to 650 nm or a trisazo compound having a sensitivity in the longer wavelength range.
Electrical characteristics generally required by a photoconductor are good chargeability, little dark decay, and low residual potential, and endurance of these characteristics during repeated use, as well as the charge generation function upon receipt of light. An organic photoconductor with a structure of function-separated laminated organic thin films, in particular, needs sufficient adhesion ability between the organic thin film and a conductive substrate, and between the organic thin films. The adhesion ability is essential to secure photoconductive characteristic, mechanical strength, and image quality. When a photoconductor has a structure provided with an undercoat layer, in which a charge generation layer is sandwiched by upper and lower organic thin films, selection of the organic binder resin to bind pigment particles in the organic layer is quite important to attain superior adhesion ability with the adjacent organic thin films. A charge generation layer cannot achieve a desired level of the electrical characteristics required by a photoconductor as described above if an organic binder resin in the charge generation layer does not have a sufficient adhesion ability with the conductive substrate or with the upper and lower organic thin films.
With miniaturization and cost reduction of a body of apparatuses such as printers in recent years, a radius of a cylindrical conductive substrate of a photoconductor is decreasing, which in turn is increasing stress in the organic photoconductor. Accordingly, more enhancement of adhesive force is required between layers in the photosensitive layer and between the cylindrical substrate and the photosensitive layer. Moreover, levels of electrical characteristics are required that cannot be achieved without provision of an undercoat layer on a substrate. In order to meet requirements of the market, an undercoat layer formed of an organic resin is increasingly employed in place of an expensive alumite layer, aiming at cost reduction. If a poly(vinyl acetal) resin that has been developed as a resin exhibiting excellent adhesion ability with a surface of a metallic substrate is used for an organic binder resin in a charge generation layer formed on a resin undercoat layer, the adhesion ability of the resin of the charge generation layer with the underlayer made of an organic resin becomes apparently insufficient.
Undercoat layers formed of organic resin may be one of two types. A type of an undercoat layer is formed of organic resin alone. Another type of an undercoat layer formed of an organic resin contains additives of fine particles of a metal oxide to adjust electric characteristics of the photoconductor by controlling a conductivity of the undercoat layer and avoiding image defects in a form of an interference fringe generated by a multiple reflection of exposure light. If a charge generation layer is formed using an organic binder resin of poly(vinyl acetal) on either type of the undercoat layer, formed of resin material containing a thermosetting resin in particular, the charge generation layer exhibits rather poor adhesion ability in comparison with a charge generation layer formed directly on a conductive substrate of aluminum.